The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to compositions and methods for removing drilling fluids from a subterranean wellbore.
During the construction of subterranean wells, it is common, during and after drilling, to place a tubular body in the wellbore. The tubular body may comprise drillpipe, casing, liner, coiled tubing or combinations thereof. The purpose of the tubular body is to act as a conduit through which desirable fluids from the well may travel and be collected. The tubular body is normally secured in the well by a cement sheath. The cement sheath provides mechanical support and hydraulic isolation between the zones or layers that are penetrated by the well. The latter function prevents hydraulic communication between zones that may result in contamination. For example, the cement sheath blocks fluids from oil or gas zones from entering the water table and polluting drinking water. In addition, to optimize a well's production efficiency, it may be desirable to isolate, for example, a gas-producing zone from an oil-producing zone. The cement sheath achieves hydraulic isolation because of its low permeability. In addition, intimate bonding between the cement sheath and both the tubular body and borehole may prevent leaks.
The cement sheath is placed in the annular region between the outside of the tubular body and the subterranean borehole wall by pumping the cement slurry down the interior of the tubular body, out the bottom and up into the annulus. The cement slurry may also be placed by the “reverse cementing” method, whereby the slurry is pumped directly down into the annular space. During the cementing process, the cement slurry is frequently preceded by a spacer fluid or chemical wash to prevent commingling with drilling fluid in the wellbore. These fluids also help clean the tubular-body and formation surfaces, promoting better cement bonding and zonal isolation. The cement slurry may also be followed by a displacement fluid such as water, a brine or drilling fluid. This fluid may reside inside the tubular body after the cementing process is complete. Hereinafter, the word “casing” will be considered to be equivalent to “tubular body.” A complete description of the cementing process and the use of spacer fluids and chemical washes is presented in the following publications.
Piot B and Cuvillier G: “Primary Cementing Techniques,” in Nelson E B and Guillot D (eds.): Well Cementing—2nd Edition, Houston, Schlumberger (2006): 459-501.
Daccord G, Guillot D and Nilsson F: “Mud Removal,” in Nelson E B and Guillot D (eds.) Well Cementing—2nd Edition, Houston, Schlumberger (2006): 143-189.
Most primary cementing operations employ a two-plug cement placement method (see FIGS. 1A-1D). After drilling through an interval to a desired depth, the drillpipe is removed, leaving the borehole 101 filled with drilling fluid 102. A casing string 103 is lowered to the bottom of the borehole, forming an annulus 104 between the casing string and the borehole (FIG. 1A). The bottom end of the casing string is protected by a guide shoe or float shoe 105. Both shoes are tapered, commonly bullet-nosed devices that guide the casing toward the center of the hole to minimize contact with rough edges or washouts during installation. The guide shoe differs from the float shoe in that the former lacks a check valve. The check valve can prevent reverse flow, or U-tubing, of fluids from the annulus into the casing. Centralizers 106 are placed along casing sections to help prevent the casing from sticking while it is lowered into the well. In addition, centralizers keep the casing in the center of the borehole to help ensure placement of a uniform cement sheath in the annulus between the casing and the borehole wall.
As the casing 103 is lowered into the well, the casing interior may fill with drilling fluid 102. The objectives of the primary cementing operation are to remove drilling fluid from the casing interior and borehole, place a cement slurry in the annulus and fill the casing interior with a displacement fluid such as drilling fluid, brine or water.
Cement slurries and drilling fluids are often chemically incompatible. Commingling these fluids may result in a thickened or gelled mass at the interface that would be difficult to remove from the wellbore, possibly preventing placement of a uniform cement sheath throughout the annulus. Therefore, a chemical and physical means may be employed to maintain fluid separation. Chemical washes 107 and spacer fluids 108 may be pumped after the drilling fluid and before the cement slurry 109 (FIG. 1B). These fluids have the added benefit of cleaning the casing and formation surfaces, which helps achieve good cement bonding.
Wiper plugs are elastomeric devices that provide a physical barrier between fluids pumped inside the casing. A bottom plug 110 separates the cement slurry from the drilling fluid, and a top plug 111 separates the cement slurry from a displacement fluid 112 (FIG. 1C). The bottom plug has a membrane 113 that ruptures when it lands at the bottom of the casing string, creating a pathway through which the cement slurry may flow into the annulus. The top plug 111 does not have a membrane; therefore, when it lands on top of the bottom plug, hydraulic communication is severed between the casing interior and the annulus (FIG. 1D). After the cementing operation, engineers wait for the cement to cure, set and develop strength—known as waiting on cement (WOC). After the WOC period additional drilling, perforating or other operations may commence.
Another purpose of a bottom plug is to scrape stationary drilling fluid or drilling fluid solids from the casing interior, leaving a clean casing interior surface and pushing the drilling fluid material out of the casing and into the annulus.
There are certain primary cementing situations where it is not possible to launch a bottom plug as a separator between the cement slurry and the fluids that have been previously pumped into the wellbore. Such operations include two-stage cement jobs and liner cementing. If a bottom plug is not present, a layer of drilling fluid and drilling fluid solids may remain along the interior casing surface. As the cement slurry passes by the casing surface, drilling fluid material may become incorporated in (or commingle with) the cement slurry, and such contamination may cause chemical and rheological difficulties.
Furthermore, as the top plug travels down the casing interior, it wipes the casing surface clean and the drilling fluid material that may accumulate below the top plug could further contaminate the cement slurry. At the end of displacement, most of this contaminated cement slurry may come to rest in the annular space between the float collar and float shoe, thereby severely compromising the mechanical properties of the cement.